VIENNA (Reuters) - A Ukrainian activist said on Thursday he was abducted and tortured by a pro-Russian faction in Crimea in an 11-day ordeal before he was released in a prisoner swap last month.

Andrei Shchekun spoke to reporters in Vienna on the sidelines of a conference on torture hosted by the Organization for Security and Co-operation's human rights arm, after being introduced by U.S. ambassador to the OSCE Daniel Baer.

Shchekun said his captors had stripped him naked, shot him with air guns, beat him, attached electrodes to his body and threatened to cut off his ear as the price for getting back a cross he was wearing when snatched on March 9.

"The world community has to act to stop what is going on in Crimea and defend those people who do not want to become part of Russia but still have to live with the crisis," he said.

Mikhail Sheremet, commander of the Crimean "self-defense" force that Shchekun claims to have been tortured by, said he had not heard of the case and dismissed accusations of torture.

"Any person can say whatever he wants to, including that he was tortured and then released - complete nonsense," he told Reuters by phone. "Members of the self-defense have had nothing to do with any Shchekuns or any other kidnappings."

The "self-defense" forces, a militia denounced by the Western-backed government in Kiev as Moscow-sponsored thugs, is comprised mainly of former servicemen and volunteers.

Russia annexed the Crimean peninsula after a March 16 referendum that western powers have rejected as illegitimate.

Shchekun described himself as a coordinator of the pro-EU Euromaidan movement who had lived in the central Crimean town of Bakhchysaray for 23 years.

He organized peaceful protests that switched to opposing the presence of Russian forces when they arrived, he said.

He said he and another activist were seized at a railway station by the Crimean self-defense forces. Two men in civilian clothes then took him - his eyes and hands bound - to a location he later discovered to be a military facility.

He said they were stripped naked for a first interrogation.

"Then two people entered the room and they didn't speak Russian but I thought they spoke a Chechen-like language, it was not the Crimean Tatar language," he said through a translator.

His abductors questioned him about suspected plans to block the March 16 referendum, links to Ukrainian nationalists and the source of his financing, he said.

His finances were also the focus of a second interrogation session. "Those who interrogated us looked like Chechens, and I think the ones in charge of the interrogations were Russians," he said.

"When given back my clothing I asked for a cross that was taken. Then someone approached me and said into my ear: 'I cannot give it back to you like that. I can only exchange it, for example for your ear.'"

After a tense minute of silence the man left. A day later a man was brought in who had been shot twice in the legs and was missing an ear, he said.

He said both military personnel and civilians with him were subjected to torture. At one stage his captors slid a knife across his skin and threatened to cut out his liver, he said.

Shchekun said he was freed on March 20 and treated at a hospital in Kherson before being brought to Kiev. He said he thought more people were being held in Crimea but could not say how many.

Before the March 16 referendum, news agency RosUkrInform quoted Crimea's separatist leader Sergei Askyonov as saying Shchekun was not kidnapped but detained by security services - not by the self-defense units - for steering unrest. Aksyonov said local pro-European activists were safe and sound, the agency added.

(Additional reporting by Gabriela Baczynska; Editing by Hugh Lawson)[You must be registered and logged in to see this link.]